1. Field of the Invention
The present invention relates to an analog switch circuit and signal attenuation employing an analog switch circuit. More specifically, the present invention relates to prevention of a switching noise in an analog switch circuit comprised in a signal attenuator for use in an audio apparatus.
2. Description of the Prior Art
FIG. 1 is a schematic diagram of an analog switch circuit which constitutes the background of the invention. Referring to FIG. 1, a transfer gate 1 comprises P-type and N-type insulated gate field-effect transistors 2 and 3. Such transistors are hereinafter referred to simply as transistors. First electrodes of the respective transistors 2 and 3 are commonly connected to an output terminal 5 and second electrodes thereof are also commonly connected to an input terminal 4. A load 6 is connected to the output terminal 5 and an input signal source 7 of an impedance lower than that of the load 6 is connected to the input terminal 4. A control signal obtained from a control input terminal 8 is applied to a gate of the N-type transistor 3 serving as a third electrode thereof and a control signal is applied, after inversion by inverter 9, to a gate of the P-type transistor 2.
When a control signal of the high level is applied to the control signal input terminal 8, the N-type transistor 3 is responsive to the control signal of the high level to be rendered conductive and the P-type transistor is responsive to the control signal of the low level obtained through inversion by the inverter 9 to be rendered conductive. In such analog signal as shown in FIG. 1, it is necessary to select the ON resistance of the transfer gate 1 to be as small as possible. To that end, it is necessary to make the gates of the P-type and N-type transistors 2 and 3 constituting the transfer gate 1 large sized as compared with those of general transistors. However, an increased size of the gate of a transistor accordingly increases a gate/source capacitance and a gate/drain capacitance and as a result such conventional P-type and N-type transistors 2 and 3 involved a disadvantage that a voltage change at gates on the occasion of turning on or off of the transfer gate 1 is leaked to the output through such capacitances, with the result that a switching noise appears on the output side.
FIG. 2 is a block diagram specifically showing a signal attenuator employing such analog switch as shown in FIG. 1. Referring to FIG. 2, the signal attenuator comprises a plurality of such analog switches 10a, 10b, . . . 10g as shown in FIG. 1, the output ends of which are connected in parallel to an output terminal 13. A load 6 including an amplifier, a speaker and the like is connected to the output terminal 13. A bias voltage source 15 is connected to a bias voltage input terminal 14 and a resistance voltage dividing circuit 11 including a plurality of resistors 11a, 11b, . . . 11f is connected between the above described bias voltage input terminal 14 and the input terminal 12. An input terminal of each of the analog switches 10a, 10b, . . . 10g is connected to one end of each of the resistors 11a, 11b, . . . 11f.
Meanwhile, in the case where the signal attenuator is employed as an electronic variable resistor serving as a volume cotroller, the respective resistance values are selected so as to provide the output for example of the A standard curve such that normally the resistor 11a has the maximum resistance value and the resistors 11b, 11c, . . . 11f has the resistance values which are smaller in succession.
A press button key 16 is a key for enabling attenuation of a signal and it is adapted that upon each depression of the key 16 a clock pulse is generated from an oscillator 17 and is applied to a counter 18. The counter 18 serves to count the number of clock pulses and the count output is applied to a decoder 19. The decoder 19 serves to decode the count output of the counter 18 and decoded outputs D.sub.0, D.sub.1, . . . D.sub.n, are applied as control signals to the respective analog switches 10a, 10b, . . . 10g. A selective one of the plurality of switches 10a through 10g is turned on in response to the presence of a respective one of control signals D.sub.0, D.sub.1, . . . D.sub.n. When the selected one of switches 10.sub.a through 10.sub.g is so turned on, the impedance of its main current path substantially decreases, thereby connecting the voltage developed at the associated common connection point between two of the resistors of resistive voltage divider 11 and the turned-on switch to terminal 13, as an output Voltage V.sub.0.
In the above described signal attenuator shown in FIG. 2, the analog switch 10g has one end of its main current path connected directly, that is by a relatively low resistance conductor compared to the resistance of any one of resistors 11.sub.a through 11.sub.f. One end of resistive circuit 11 also is connected to bias voltage input terminal 14. Accordingly, it is necessary to make small the ON resistance of the analog switch 10g in order to decrease crosstalk between switch 10g and the other switches 10a through 10f. Accordingly, the gate size of the P-type and N-type transistors constituting the analog switch 10g need be of a size larger than the gate size of the transistors constituting the other analog switches 10a, 10b, . . . 10f. However, as the gates of the transistors are made larger, the gate/source capacitance and the gate/drain capacitance accordingly becomes larger in the same manner as described previously in conjunction with FIG. 1 and therefore a voltage change at the gate is leaked through such capacitances to the output on the occasion of switching of the analog switch, with the resultant disadvantage that a switching noise appears on the output side.